Catalysts for purifying vehicle exhaust gas are composed of a catalytic metal such as platinum, palladium, or rhodium, and a co-catalyst for enhancing the catalytic action of such metal, both supported on a catalyst support made of, for example, alumina or cordierite. As such a co-catalyst material are used cerium oxide-containing materials, which have the properties of absorbing oxygen under the oxidizing atmosphere and desorbing oxygen under the reducing atmosphere, originated in ceric oxide, i.e., oxygen absorbing and desorbing capability. With this oxygen absorbing and desorbing capability, the cerium oxide-containing materials purify noxious components in exhaust gases such as hydrocarbons, carbon monoxide, and nitrogen oxides at excellent efficiency. As such, large quantities of the cerium oxide-containing materials are used as a co-catalyst.
It is most critical for activating the function of such cerium oxide-containing co-catalyst material to keep the co-catalyst at a high temperature. Low temperature of the exhaust gas, for example at engine start-up, will result in low purifying efficiency. Vehicle manufacturers are presently trying to solve this problem by placing the catalyst system close to the engine for introducing hot exhaust gas right after its emission from the engine into the catalyst system. There is also a demand for co-catalyst materials that are activated at lower temperatures.
In general, efficiency of exhaust gas treatment with a catalyst is proportional to the contact area between the active phase of the catalyst and the exhaust gas, and to the oxygen absorbing and desorbing capability of the co-catalyst material, such as ceric oxide. Thus the co-catalyst material is required to have a sufficiently large specific surface area and a sufficiently high oxygen absorbing and desorbing capability, as well as high activity at lower temperatures.
For solving these problems, JP-7-61863-B proposes a method for obtaining a ceric oxide having good heat resistance, including precipitating ceric hydroxide in a reaction medium at pH 6 to about pH 10, treating the resulting precipitate in an autoclave at 100 to 350° C., and calcining at 300 to 1000° C., to thereby obtain ceric oxide. However, the heat resistance of the resulting ceric oxide represented by the specific surface area after calcination at 900° C. is 15 m2/g, which is not sufficient.
JP-2001-89143-A, JP-2000-281343-A, JP-2789313-B, and JP-2000-l28537-A propose cerium-containing oxides having improved oxygen storage capacity (OSC) However, all of these oxides are composite oxides containing ceric oxide having one or more other elements solid-solutioned therein, and are not high-purity ceric oxides.
JP-3-24478-B, JP-3-24411-B, and JP-2537662-B propose methods for preparing ceric oxide including refluxing an aqueous solution of ceric nitrate, separating the resulting hrydrolysate by filtering, washing, drying, and calcining, to thereby prepare ceric oxide. The ceric oxide thus obtained, however, has low heat resistance represented by a specific surface area after calcination at 900° C. for 5 hours of as low as not higher than 10 m2/g.